We study the structure and use of the personality categories humans employ to describe and explain the individual differences they see about them. We exploit two fundamental principles of natural categories, breadth and hierarchical level, to organize our knowledge about personality traits and states. We plan to extend our analyses of the distinction between the categories Trait and State by examining the extent to which their membership, and attribute salience, varies as a function of context. In analyses of the use of trait categories in descriptions of oneself and others, we have found a robust preference for the superordinate level. We will not test a series of hypotheses about the causes of this preference, and try to shift the preferred level by manipulating perceiver goals, task purpose, target likeability and familiarity, and perceiver affect. We propose to study the causal theories underlying traits at different levels. People see broad traits as the cause of narrow ones but not the reverse, and different types of causes (Necessity, Sufficiency, and Association) are associated with different types of traits (Temperament, Capacities, and Motives). Now we will extend these causal analyses and investigate the influence of a variety of contextual factors, including Actor-Observer points of view. Ultimately, the structure of trait categories must be understood at the level of attributes or features, and we propose four covergent methods for eliciting the attributes of traits at different hierarchical levels. In addition, we propose to conduct some comparative- validity studies to demonstrate that our conception of personality traits as prototype-based categories can be profitably applied to personality assessment. Over-all, our analyses should have important implications for future studies of persons perception and trait attribution, for the development of trait taxonomies, and for the measurement of individual differences.